1. Field
The present invention relates to an optical transmission apparatus, a subscriber apparatus and an optical communication system, and in particular, relates to an optical communication system that performs optical communication by means of an optical network including a plurality of subscriber apparatuses and an optical transmission apparatus that performs communication with the plurality of subscriber apparatuses, and an optical transmission apparatus and a subscriber apparatus constituting the optical communication system.
2. Description of the Related Art
In recent years, with widespread use of the Internet, information communication networks have penetrated into families and enterprises and optical subscriber system networks are evolving to provide faster large-capacity services. As a concrete optical subscriber system, for example, a PON (Passive Optical Network) system is widely being adopted.
FIG. 12 is a diagram showing the configuration of a PON system. A PON system 6 includes an OLT (Optical Line Terminal) 6a set up on the office side, ONUs (Optical Network Units) #1 to #n set up on the subscriber side, and an optical passive-element star coupler 6b that performs optical demultiplexing/multiplexing.
The OLT 6a and the ONUs #1 to #n are 1-to-n connected by an optical fiber F via the star coupler 6b to perform optical packet communication between the OLT 6a and the ONUs #1 to #n. The wavelength used is generally 1.31 μm in the up direction (ONU→OLT) and 1.49 μm in the down direction (OLT→ONU).
While down signals are transmitted continuously, up signals are transmitted in burst mode so that up signals from different ONUs should not collide with each other. Since the transmission distance between the OLT 6a and the ONUs #1 to #n is different from ONU to ONU, optical packets of different levels will be received.
Here, when a system is being constructed, the OLT 6a transmits a delay measurement instruction optical packet to each ONU. The ONU that receives a delay measurement instruction optical packet and recognizes that the optical packet is addressed to the ONU transmits a return packet to the OLT 6a. 
Based on the reception timing of return packets, the OLT 6a recognizes the propagation delay time (transmission distance) between the OLT 6a and each ONU and decides the transmission timing when the ONUs #1 to #n transmit packets so that such packets should not collide with each other. Then, the OLT 6a notifies each ONU of the transmission timing decided for the ONU and the ONU transmits packets to the OLT 6a based on the notified transmission timing to realize transmission in burst mode without mutual collision in the up direction.
A technology to determine, when a collision of a data cell transmitted from an ONU and a distance measurement cell transmitted from another ONU is detected, the transmission distance between the ONU that transmitted the distance measurement cell and an OLT is proposed by Japanese Patent Application Laid-Open No. 2003-18174 as conventional technology.
The function to measure the propagation delay time as described above is generally called ranging. In the ranging, the propagation delay time between the OLT 6a and the ONUs #1 to #n is determined by recognizing which position of a delay measurement window a return packet from the ONUs #1 to #n enters after a delay measurement instruction optical packet is transmitted from the OLT 6a. The delay measurement window is a time range from the minimum value to the maximum value of the reception timing when return packets are received.
FIG. 13 is a diagram showing a state of ranging. The OLT 6a transmits a delay measurement instruction optical packet s1 containing an identifier addressed to the ONU #1. The delay measurement instruction optical packet s1 reaches all of the ONUs #1 to #n, but the ONU #1 recognizes that the delay measurement instruction optical packet s1 is addressed to the ONU #1 and thus, only the ONU #1 captures the delay measurement instruction optical packet s1 and transmits a return packet p1 to the OLT 6a. Then, the OLT 6a recognizes the reception timing of the return packet p1 entering a delay measurement window W.
Similarly, the OLT 6a transmits a delay measurement instruction optical packet s2 containing an identifier addressed to the ONU #2 and the ONU #2 transmits a return packet p2 to the OLT 6a. Then, the OLT 6a recognizes the reception timing of the return packet p2 entering the delay measurement window W.
Since the return packet p1 reaches the OLT 6a earlier than the return packet p2 in FIG. 13, it is clear that the propagation delay time related to the ONU #1 and the OLT 6a is shorter than that related to the ONU #2 and the OLT 6a (The ONU #1 is positioned nearer to the OLT 6a than the ONU #2).
In this manner, the OLT 6a transmits a delay measurement instruction optical packet to each ONU and recognizes the propagation delay time related to the OLT 6a and each of the ONUs #1 to #n based on the reception timing of the return packet returned from each ONU in the delay measurement window W to decide the transmission timing for each of the ONUs #1 to #n so that information packets transmitted by each ONU during system operation should not collide with each other.
However, in conventional ranging, only the reception timing of a return packet from an ONU entering a delay measurement window is detected and the state of an optical level of a return packet transmitted from the ONU is not determined.
Thus, the state of light transmission from an ONU and the state of transmission paths cannot be detected during ranging. For example, if an ONU being ranged emits light abnormally or an optical loss arises due to a failure of an optical transmission path between an ONU and an OLT (such as an occurrence of optical loss by using an optical fiber generally made available), quality of such optical propagation cannot be determined during execution of ranging.
Since the above ranging is generally performed when an optical fiber is deployed to construct a PON system, it is preferable that quality of optical propagation between ONUs and the OLT be recognizable, as well as a decision of the transmission timing of ONUS, to improve maintainability.
The present invention has been developed in view of the above circumstances and an object thereof is to provide an optical communication system capable of determining quality of an optical transmission state between a station and subscribers by measuring the propagation delay time and also detecting the optical level of packets transmitted from subscribers during execution of ranging.